Light source system

ABSTRACT

The present disclosure relates to a light source system suitable for use in a time of flight camera. The light source system includes a light source, such as a laser, and a driver arranged to supply a drive current to the light source to turn the light source on to emit light. The driver includes a capacitor to store energy and then discharge to generate the drive current, and the driver is integrated into a semiconductor die on which the light source is mounted. Consequently, the driver includes within it the source of energy for the drive current and the light source and driver are very close together, meaning that the light source may be turned on and off very quickly with a relatively large drive current, in order to output a high optical power, short duration light pulse.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.16/780,735, filed Feb. 3, 2020, which is hereby incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to a light source system, which may beused, for example, as the light source in a time-of-flight camerasystem.

BACKGROUND

Time-of-flight (ToF) camera systems are range imaging systems thatresolve the distance between the camera and an object by measuring theround trip time of a light signal emitted from the ToF camera system.The systems typically comprise a light source (such as a laser or LED),a light source driver to control the emission of light from the lightsource, an image sensor to image light reflected by the subject, animage sensor driver to control the operation of the image sensor, opticsto shape the light emitted from the light source and to focus lightreflected by the object onto the image sensor, and a computation unitconfigured to determine the distance to the object by determining theamount of time between an emission of light from the light source and acorresponding reflection from the object.

ToF camera systems may measure distances ranging from a few centimetresto 100s or 1000s of metres. Given the high speed of light, a timedifference of only 16.66 ns between an emission of light and receptionof reflected light corresponds to an object 2.5 m from the camerasystem. Therefore, ToF camera systems require high levels of temporalprecision and control in order to measure distances accurately.

SUMMARY

The present disclosure relates to a light source system suitable for usein a time of flight camera. The light source system includes a lightsource, such as a laser, and a driver arranged to supply a drive currentto the light source to turn the light source on to emit light. Thedriver includes a capacitor to store energy and then discharge togenerate the drive current, and the driver is integrated into asemiconductor die on which the light source is mounted. Consequently,the driver includes within it the source of energy for the drive currentand the light source and driver are very close together, meaning thatthe light source may be turned on and off very quickly with a relativelylarge drive current, in order to output a high optical power, shortduration light pulse.

In a first aspect of the present disclosure, there is provided a lightsource system comprising: a light source; and a semiconductor diecomprising an integrated first driver, wherein the light source ismounted on the semiconductor die and the first driver is coupled to thelight source and configured to control supply of a first drive currentto the light source for controlling operation of the light source,wherein the first driver comprises a first capacitor for storingelectrical energy for use in generating the first drive current.

The first driver may be configured to operate in: a charging state,during which the first capacitor stores charge received from a powersupply; and an emission state, during which the first capacitordischarges to generate the first drive current, which is supplied to thelight source to turn the light source on.

The first capacitor may be set to any suitable size (i.e., capacitance),depending on the size of first drive current required and/or theduration of first drive current required and/or the duty cycle ratiobetween charging state and emission state.

The first driver may further comprise a first switch configured to:during the emission state, close a first drive current circuitcomprising the first capacitor and the light source to carry the firstdrive current between the light source and the first driver; and duringthe charging state, open the first drive current circuit during thecharging state to stop the supply of first drive current to the lightsource.

The first switch may be coupled between a cathode terminal of the lightsource and a reference voltage of the first driver, and wherein thefirst capacitor is coupled between an anode terminal of the light sourceand the reference voltage of the first driver.

The light source system may further comprise a controller configured tocontrol the first switch, so as to control switching of the first driverbetween the charging state and the emission state.

The first switch may comprise a first transistor, wherein the controlleris configured to control operation of the first switch by controlling afirst transistor driver signal at a gate/base terminal of the firsttransistor.

The light source system may further comprise a turn-on pre-driver,wherein the controller is configured to turn on the first transistor byapplying a turn-on signal to the gate/base terminal of the firsttransistor using the turn-on pre-driver in order to transition the firstdriver from the charging state to the emission state.

The light source system may further comprise a voltage regulator coupledto the first capacitor and the turn-on pre-driver, wherein the voltageregulator is configured to: receive energy from the first capacitor; andsupply a regulated voltage to the turn-on pre-driver at least duringtransition of the first driver from the charging state to the emissionstate.

The light source system may further comprise a turn-off pre-driver,wherein the controller is configured to turn off the first transistor byapply a turn-off signal to the gate/base terminal of the firsttransistor using the turn-off pre-driver in order to transition thefirst driver from the emission state to the charging state.

To transition the first driver from the emission state to the chargingstate, the controller may be configured to apply both the turn-on signaland the turn-off signal to the gate/base terminal of the firsttransistor for a first period of time, and then apply only the turn-offsignal to the gate/base terminal of the first transistor for a secondperiod of time.

The light source system may further comprise a photodetector arranged todetect light emitted from the light source, wherein the light sourcesystem is further configured to charge the first capacitor only if thephotodetector detects light emitted from the light source during thepreceding emission state.

The light source may comprise a laser, for example a vertical-cavitysurface-emitting laser.

The light source may comprise at least one terminal of a first polarityon a first surface of the light source and at least one terminal of asecond polarity on a second surface of the light source, wherein thefirst driver is coupled to the at least one terminal of the firstpolarity and the at least one terminal of the second polarity such thatthe first drive current can flow through the light source to turn on thelight source.

The second surface of the light source may be affixed to a first surfaceof the semiconductor die, and wherein the first driver is coupled to theat least one terminal of the first plurality by a first plurality ofbonding wires.

The light source may comprise a first terminal of a first polarity, asecond terminal of the first polarity and at least one third terminal ofa second polarity, and wherein the first driver is coupled to the firstterminal and the third terminal such that the first drive current canflow between the first terminal and the third terminal to turn on thelight source, and wherein the semiconductor die further comprises anintegrated second driver coupled to the second terminal and the thirdterminal, wherein the second driver is configured to control supply of asecond drive current to the light source such that the second drivecurrent can flow between the second terminal and the third terminal toturn on the light source, wherein the second driver comprises a secondcapacitor for storing electrical energy for use in generating the seconddrive current.

The first terminal and the second terminal may be are arranged on thelight source such that they are substantially symmetrical about a planeof symmetry, wherein the first driver and the second driver are arrangedwithin the die such that they are substantially symmetrical about theplane of symmetry.

In a second aspect of the present disclosure, there is provided a timeof flight camera system comprising: a light source system comprising: alight source for emitting light towards an object to be imaged; and asemiconductor die comprising an integrated first driver, wherein thelight source is mounted on the semiconductor die and the first driver iscoupled to the light source and configured to control supply of a firstdrive current to the light source for controlling operation of the lightsource, wherein the first driver comprises a first capacitor for storingelectrical energy for use in generating the first drive current; and aphotodetector for receiving light reflected from the object to theimaged.

In a third aspect of the present disclosure, there is provided a driverfor coupling to a light source to drive the light source, the drivercomprising: at least one capacitor for storing charge; a controllableswitch for switching the driver between a charging state and an emissionstate; a turn-on pre-driver coupled to the controllable switch, whereinthe turn-on pre-driver is configured for use in controlling thecontrollable switch when transitioning from the charging state to theemission state; and a voltage regulator coupled to the at least onecapacitor and the turn-on pre-driver and configured to supply aregulated voltage to the turn-on pre-driver, wherein the driver isconfigured such that during the charging state the at least onecapacitor stores charge and during the emission state the at least onecapacitor dischargers to supply a drive current to the light source toturn the light source on.

The driver may be configured for coupling to a power supply such thatduring the charging state, the at least one capacitor stores chargereceived from the power supply.

DRAWINGS

Aspects of the present disclosure are described, by way of example only,with reference to the following drawings, in which:

FIG. 1 shows an example representation of a light source system, inaccordance with an aspect of the present disclosure;

FIG. 2A shows a view of the arrangement of the VCSEL 110 mounted on thesemiconductor die 120 of the light source system of FIG. 1 ;

FIG. 2B shows a further view of the arrangement of the VCSEL 110 mountedon the semiconductor die 120 of the light source system of FIG. 1 ;

FIG. 3 shows an example graph representing a drive current for the VCSELand optical power of light output from the VCSEL of the light sourcesystem of FIG. 1 ;

FIG. 4 shows a schematic representation of details of an exampleimplementation of the light source system of FIG. 1 ;

FIG. 5 shows an example representation of the first drive currentgenerated by the first driver represented in FIG. 4 ;

FIG. 6 shows a schematic representation of details of a further exampleimplementation of the light source system of FIG. 1 ; and

FIG. 7 shows a schematic representation of details of a further exampleimplementation of the light source system of FIG. 1 .

DETAILED DESCRIPTION

Many factors may affect the precision with which a ToF camera system canmeasure distance to an object. One of those factors is the nature of thelight emitted from the light source. The accuracy with which the systemknows the moment of light emission may affect how accurately the systemcan determine distance to the object. For example, if the timedifference between the moment the system believes it emitted light andthe moment at which the reflected light was received is 20 ns, an objectdistance of about 3 m will be determined. However, if the emission oflight actually took place about 0.4 ns after the system thought it tookplace, the true time difference between light emission and lightreflection is in fact about 19.6 ns, which equates to an object distanceof 2.94 m. For many safety critical applications, an error of this sizemay be significant.

The inventors have realised that if the light source is controlled toemit a very short pulse of light (in the order of 100 ps or less), theprecision of the ToF camera system may be improved. A relatively longlight emission pulse may render it unclear whether reflected lightcorresponds to photons emitted at the very start, middle or very end ofthe light emission pulse. To address this, more samples may be gatheredto help de-convolve the shape of the laser, but this slows down thesystem and consumes more power. However, if the light emission pulse isvery short, such as less than 100 ps, this represents a maximumuncertainty of 0.1 ns, which may be acceptable for most applications.Consequently, the time between emission of light and correspondingreflection of light may be measured with more certainty.

To achieve very fast turn on of the light source, the inventors havedetermined that a high current light source driving signal may bebeneficial. Driving the light source with a relatively high, shortduration current pulse should shorten the duration of the output lightpulse, and increase the peak optical power output. The inventors haverealised that this higher peak optical power may bring a further benefitin improving the precision of the ToF camera system. In particular, anumber of safety regulations limit the average optical power emittedfrom ToF camera systems with no limit on the peak optical power output.By reducing the light emission duration, a system may emit higheroptical power whilst staying within safety regulations. Emitting lightwith higher peak optical power may be beneficial for improving the rangeand precision of the ToF camera system.

However, there are many challenges in driving a light source to emit avery short, high optical power light pulse. A very short, high currentdriving signal may be required for driving the light source, which mayput considerable demands on the power supply/source providing thedriving current and may affect other electrical components that are alsousing that power supply. For example, if the ToF camera system isintegrated in a mobile phone, drawing a relatively high current for avery short period of time, without affecting any other components orfunctionality of the mobile phone, may require a very high quality,costly power system within the mobile phone. Furthermore, if the ToFcamera system operates with relatively high current pulses, if a systemfault/failure develops, the relatively high currents may present asafety risk to the ToF camera system and/or nearby devices/componentsand/or operators of the ToF camera system.

In addition to this, to achieve such short light emission pulses, thelaser source should be turned on and off rapidly and with precision.Inherent resistances and inductances in the light source driver circuitsnot only contribute to electrical losses (thereby increasing the amountof current needed to achieve a particular optical power output from thelight source), but also slows circuit transitions between currentlevels, thereby slowing turn on and turn off.

With all of these challenges in mind, the inventors have devised a lightsource system that is suitable for use in ToF camera systems. In thelight source system, the light source driver is integrated into asemiconductor die and includes a capacitor as the energy source forgenerating the current to drive the light source. The driver is designedsuch that the capacitor may be gradually charged using a relatively lowcurrent power supply/source, and then rapidly discharged to providemost, if not all, of the energy required for the current driving thelight source. Furthermore, the light source is mounted on, or stackedon, the driver such that the physical distance between the driver andthe light source is minimised. By minimising the distance between thedriver and the light source, the physical length of the current pathbetween the driver and the light source may be reduced, thereby reducingelectrical resistance and inductance. Furthermore, by using a capacitorto supply the majority, if not all, of the energy required for thedriver current, the physical size of the circuit loop carrying thedriver current may be reduced compared with using a separate/externalpower unit, which even further reduces resistance and inductance. Afurther benefit of using a capacitor in this way is that if a systemfault/failure develops, there is only a limited amount of energyavailable for generating the high current, which should improve overalldevice safety.

FIG. 1 shows an example representation of a light source system 100,suitable for use in a ToF system, in accordance with an aspect of thepresent disclosure. The light source system 100 comprises a light source110, in this example a vertical-cavity surface-emitting laser (VCSEL),and a semiconductor die 120, such as a CMOS silicon die, or a GaN die,or a GaAs die, etc. Throughout this disclose, the light source 110 shalltypically be referred to as a VCSEL, although it will be appreciatedthat any other suitable type of light source may alternatively be used,such as other types of laser or any suitable type of LED.

The VCSEL comprises a first surface 112, from which light is emitted,and a second surface 114, which opposes the first surface 112. The firstsurface 112 comprises a first terminal 116 and a second terminal 118both of a first polarity, the first polarity in this example being theanode of the VCSEL diode. The second surface 114 comprises one or morethird terminals of a second polarity, the second polarity in thisexample being the cathode of the VCSEL diode. The VCSEL 110 is mountedon, or stacked on, the semiconductor die 120. The second surface 114 ofthe VCSEL 110 is mounted on a first surface 122 of the semiconductor die120 and bonded to the first surface 122 of the semiconductor die 120using bonding material 150 and electrical interconnects, in such a wayas to form an electrical connection between the one or more thirdterminals of the VCSEL 110 and the first and second drivers 130 and 140.Any suitable bonding material 150 and bonding techniques may be used forthis, for example an epoxy or eutectic bond.

The semiconductor die 120 comprises an integrated first driver 130 andan integrated second driver 140. The first driver 130 is configured tocontrol supply of a first drive current 136 to the VCSEL 110 forcontrolling operation of the VCSEL 110 (i.e., for controlling lightemission from the VCSEL 110). The second driver 140 is configured tocontrol supply of a second drive current 146 to the VCSEL 110 forcontrolling operation of the VCSEL 110 (i.e., for controlling lightemission from the VCSEL 110). The first driver 130 comprises a firstswitch 134, which in this example is a first FET, for controlling theflow of the first drive current 136, and the second driver 140 comprisesa second switch 146, which in this example is a second FET, forcontrolling the flow of the second drive current 146. The first driver130 is electrically coupled to the first terminal 116 of the VCSEL 110by one or more first bond wires 152 (although alternatively any othersuitable form of electrical coupling may be used, for example dependingon the design of the VCSEL 110). The second driver 140 is electricallycoupled to the second terminal 118 of the VCSEL 110 by one or moresecond bond wires 154 (although alternatively any other suitable form ofelectrical coupling may be used, for example depending on the design ofthe VCSEL 110). When a sufficiently large first drive current 136 and/orsecond drive current 146 flows through the VCSEL 110, the VCSEL 110 willturn on and be excited to lase and therefore emit light. When no drivecurrent, or insufficient drive current, flows through the VCSEL 110, nolight should be emitted and the VCSEL is effectively turned off. Furtherdetails of the operation of the first driver 130 and the second driver140 shall be given later in this disclosure.

FIGS. 2A and 2B show different views of the arrangement of the VCSEL 110mounted on the semiconductor die 120. As can be seen, in this examplethere are eight first bond wires 152 and eight second bond wires 154,although any number of first bond wires 152 and second bond wires 154may be used (for example, one, two, three, etc). It may be preferable touse the largest number of first bond wires 152 and second bond wires 154possible for a given size of first terminal 116 and second terminal 118.By doing so, resistance for the first drive current 136 and the seconddrive current 146 may be minimised, thereby improving the powerefficiency of the light source system 110 and maximising the amount ofdrive current delivered to the VCSEL 110.

FIG. 3 shows graphs representing drive current (labelled VCSEL Current)and optical power of light output from the VCSEL 110. It can be seenthat in this example, a drive current pulse duration of about 290 ps maybe created by the drivers 130, 140, which may excite a light pulseemission from the VCSEL 110 with an effective duration of about 30 ps.It will be appreciated that the drive current pulse duration is longerthan the light pulse duration largely as a consequence of lasing delaysin the VCSEL 110. It will also be appreciated that this is merely onenon-limiting example of drive current and light pulse duration, and thatthe light source system 100 may be configured to operate with differentdurations.

It may be counterintuitive to mount the VCSEL 110 on the integrateddriver circuits 120, particularly when the drive currents are intendedto be relatively high (for example, >1 A, or >3 A, or >5 A, or >8 A,or >10 A) owing to thermal dissipation difficulties. The VCSEL 110 andthe drivers 130 and 140 are likely to generate significant heat when thedrive currents are flowing, which should ideally be quickly andeffectively dissipated in order to avoid device damage and degradation.It might have been expected that stacking the VCSEL 110 on top of theintegrated drivers 130 and 140 would be likely to make heat dissipationconsiderably more difficult.

However, the inventors have recognised that by mounting the VCSEL 110 onthe semiconductor die 120 which includes the integrated drivers 130,140, the physical distance between the VCSEL 110 and the drivers 130,140 may be minimised. Consequently, a first drive current circuit formedby the VCSEL 110, the first capacitor 132 and the first switch 134 tocarry the first drive current 136 may be significantly smaller thanother arrangements (for example, arrangements where the VCSEL 110 andthe first driver 130 are mounted side-by-side on a PCB substrate withbonding wires carrying current both to and from the VCSEL 110).Likewise, the same is also true of the size of the second drive currentcircuit formed by the VCSEL 110, the second capacitor 142 and the secondswitch 144. This reduction in the size of circuits carrying therelatively high drive currents may reduce the resistance and inductanceof the circuit, which may reduce circuit losses and increase the speedwith which the drive currents, and therefore the VCSEL 110, can beturned on and off. This enables the drive currents to be generated as avery short duration pulse, resulting in a very short duration pulse oflight from the VCSEL 110. Thus, by implementing the stacking arrangementrepresented in FIGS. 1, 2A, 2B, a shorter duration (for example <200 ps,or <150 ps, or <100 ps, or <80 ps, or <50 ps), higher optical powerpulse of light may be output by the VCSEL 110. As explained earlier, inorder to achieve this relatively short duration optical output from theVCSEL 110, a slightly longer duration drive current pulse may berequired, for example to achieve a 30 ps light pulse, a 300 ps durationfirst drive current 136 may be required.

Not only may this improve the precision of a ToF system that uses thelight source system 100, it has unexpectedly been realised that becauseof the short duration of current achieved by this arrangement, heatdissipation may not in fact be as significant a problem as mightinitially have been thought. ToF systems may operate by emitting lightfor a period of time and then turning off for a period of time. Forexample, a typical light emission duration for a SPAD direct ToF systemmay be a pulse duration of about 1-3 ns every 0.2 μs, representing aduty cycle ratio of about 1:100 for light emission:no light emission. Atypical light emission duration of a continuous wave (CW) indirect ToFsystem may be about 100-200 ns every 2 ms, representing a duty cycleratio between about 1:20 to 1:10. However, in the present disclosure,because such a short, high optical power light pulse has been achieved,a duty cycle of at least 1:1,000, such as 1:5,000, or 1:10,000, may beimplemented. For example, the approximately 30 ps light pulserepresented in FIG. 3 corresponds to a duty cycle ratio of about1:30,000. Consequently, there is a relatively very long period of timeduring which heat is not being generated by the light source system 100,during which time the heat generated during the emission part of thecycle may gradually dissipate into the surrounding environment and intothe body of the die (and any other material coupled to the die).

The VCSEL 110 in the example represented in FIGS. 1, 2A and 2B has twoterminals 116 and 118 of the same polarity, arranged to be substantiallysymmetrical about a plane of symmetry. In this example, the plane ofsymmetry extends perpendicular to the first and second surfaces 112, 114of the VCSEL 110, roughly through the middle of the first and secondsurfaces 112, 114, such that the first terminal 116 is on one side ofthe plane of symmetry and the second terminal 118 is its mirror image onthe other side of the plane of symmetry. The inventors have realisedthat it is possible to layout within the die 120 the components of thefirst driver 130 and the second driver 140 in such a way that they aresubstantially, or approximately, symmetrical about the plane of symmetry(as represented in FIG. 1 ). Consequently, the direction of thepath/loop of the first drive current 136 is substantially opposite tothat of the second drive current 146 (for example, the path of the firstdrive current 136 may be clockwise and the path of the second drivecurrent 146 may be anticlockwise). As a result of this, EM radiationgenerated by one of the current paths/loops may be substantially, or atleast partially, cancelled by EM radiation generated by the othercurrent path/loop, particularly at far-field. Thus, RF emissions fromthe light source system 100 (caused by the light source system 100generating pulses of light at RF frequencies) may be reduced and keptacceptably low, even when the drive currents are relatively high. Thismeans that the light source system 100 should not negatively affectother nearby electrical devices/components, or breach RF emissionslegislation. It will be appreciated that whilst this symmetricalarrangement may have benefits, such an arrangement is not essential andthe first driver 130 and second driver 140 may be relatively arranged inany other suitable way.

FIG. 4 shows a schematic representation of example details of the lightsource system 110. The representation provides further details of anexample implementation of the first driver 130, to help explain theoperation of the first driver 130 in order to control supply of thefirst drive current 136 to the VCSEL 110. In this example, the firstdriver 130 further comprises a voltage regulator 410 (which may beimplemented in any suitable way known to the skilled person), a turn-onpre-driver 420, a turn-off pre-driver 422 and a FET 440 which acts asthe first switch 134. The light source system 430 also comprises acontroller 430 configured to control switching of the first driver 130between a charging state and an emission state. The control may beimplemented in any suitable way, for example as logic integrated withinthe semiconductor die 420 or elsewhere, or using a microcontrollerconfigured to operate as described below, or any other form of processorsuitably configured to operate as described below. Likewise, the turn-onpre-driver 420 and turn-off pre-driver 422 may be implemented in anysuitable way known to the skilled person, for example as buffers and/ordigital buffers and/or amplifiers.

The first driver 130 is coupled to a power supply/source PVDD and PVSS,which may be a relatively low current power supply (for example, thefirst driver circuit 130 may draw less than 50 mA, or less than 30 mA,such as a typical average current of less than 20 mA, from the powersupply). When the first driver 130 is in the charging state, the FET 440is switched off (i.e., the first switch 134 is open), such that thecurrent flowing through the VCSEL 110 is 0 A, or substantially 0 A(i.e., sufficiently low that there is no risk of the VCSEL 110 emittingany light). The power supply may be any suitable power supply to whichthe first driver 130 may be coupled. For example, if the light sourcesystem 100 is included as part of a larger device/system (such as amobile device), the power supply may be the power supply of that largerdevice system (such as the battery of the mobile device). During thecharging state, the first capacitor 130 is gradually charged by thepower supply.

When a light pulse emission is desired, the first driver 130 may betransitioned from the charging state to the emission state. To controlthis transition, the controller 430 may use control lines 434 and/or 436to control operation of the FET 440. For example, the controller 430 mayreceive an instruction via its input line 432 (which may take anysuitable form, for example it may be an LVDS differential signal) tostart a light pulse emission from the VCSEL 110. The controller 430 maythen drive the control line 434 so that the turn-on pre-driver 420applies a turn-on signal to the gate of the FET 440 in order to turn onthe FET 440 (i.e., close the first switch 134). In this example, theturn-on signal is a voltage signal that exceeds the turn-on thresholdvoltage of the FET 440. The turn-on pre-driver 420 is used in thisexample because the first drive current 136 that will flow through theFET 440 when it is turned on should be relatively large (for example,between about 5 A and 12 A, such as >8 A, or >10 A), so the FET 440should be a relatively high power FET 440. Most controllers may not becapable of supplying a sufficiently large drive signal to the FET 440 toturn it on, or at least may not be capable of supplying a sufficientlylarge drive signal to turn the FET 440 on quickly enough to achieve aquick transition from off to on. Therefore, the turn-on pre-driver 420effectively functions to increase the signal set by the controller 430on the control line 434 to a level sufficient to drive the FET 440 toturn on quickly.

In this arrangement, the power drawn by the turn-on pre-driver 420 andturn-off pre-driver 422 is supplied by the voltage regulator 410, whichin turn draws the majority, if not all, of its power from the capacitor132 discharging. By arranging it in this way, most, if not all, of thecurrent required to switch the first driver 130 between the charging andemission states is kept within the first driver 130 and is not drawnfrom elsewhere, such as the power supply PVDD and PVSS. Some of thebenefits of drawing the majority, if not all, required energy from thecapacitor 132, rather than external sources, is explained earlier withreference to the first drive current 136. Furthermore, if the voltageregulator 410 and the pre-drivers are also integrated into the die 120,they may all be relatively close to each other and to the capacitor 132,further increasing switching speed and reducing losses.

When the FET 440 is turned on (i.e., the first switch 134 is closed),the first driver 130 is in the emission state and the first drivecurrent circuit is closed such that the first drive current 136 flowsbetween the VCSEL 110.

It can be seen that in this example arrangement of the first driver 130,the first capacitor 132 is coupled between an anode terminal of theVCSEL 110 and a reference voltage (ground in this example, but thereference voltage could alternatively be any other suitable voltagelevel) of the first driver 130 (i.e., one terminal of the capacitor 132is coupled to the VCSEL 110 and the other terminal of the capacitor 132is coupled to the reference voltage). Whilst this particularconfiguration is not essential, it has a benefit that the VCSEL 110 canbe driven by positive voltages within the first driver 130, such thatdriving the VCSEL 110 does not require the generation of negativevoltages within the first driver 130. This may be beneficial to simplifyoperation of the first diver 130 and enables the components of the firstdiver 130 to be integrated in the semiconductor die 120 using, forexample junction isolation, and not require dielectric isolation.

FIG. 5 shows an example representation of first drive current 136flowing through the first drive current circuit. During the emissionstate, the first capacitor 132 discharges to generate the first drivecurrent 136. Consequently, the majority, if not all, of the energyrequired for the first drive current 136 is supplied by the firstcapacitor 132 during the drive time, such that the draw on the powersupply PVDD and PVSS is very low (for example <5%, or <1%, of the firstdriver current 136 may be drawn from the power supply), if noteffectively zero. As such, the majority, if not all, of the first drivecurrent 136 is kept within the first driver 130 (i.e., no current, or nosignificant current, is drawn from sources elsewhere), within arelatively small first drive current circuit. In one alternativeimplementation, a further switch may be used to isolate the first driver130 from the power supply when the first diver 130 is in the emissionstate, although in most implementations this should not be necessarysince the first capacitor 130 provides so much energy during theemission state that the draw on the power supply is insignificant.

By utilising the first capacitor 132 in this way, the first capacitor132 may be gradually, slowly charged by the power supply during thecharging state, which should not affect the power supply in anysignificant way. The first capacitor 132 may then discharge to generatethe first drive current 136 such that the power supply may beeffectively, or entirely, unaffected by the relatively high first drivecurrent 136. Consequently, it may be possible for the power supply to beof a standard specification, thereby minimising costs, and may be usedby other components/systems without being affected by the light sourcesystem 100. Furthermore, because the first drive current 136 isgenerated effectively entirely by the integrated first capacitor 132acting as a source of power, the first current driver circuit is keptrelatively small (compared with the case where the first drive current136 is drawn from an external power supply), which reduces delays in thefirst drive current 136 commencing and increases the speed of operation.

During the emission state, the first driver 130 may be configured suchthat the first capacitor 132 may be completely, or only partially,discharged in the process of generating the first drive current 136. Insome implementations, generating the first drive current 136 may resultin only a partial discharge of the charge stored on the first capacitor132, such that the voltage across the first capacitor 132 is reduced(for example, by a few volts), but there is still a non-zero voltageacross the first capacitor 132 at the end of the emission state. In thiscase, the voltage across the capacitor 132 may be monitored before,during and/or after the emission state, such that the amount of firstdrive current 136 supplied to the VCSEL 110 during the emission statemay be determined. The size of the first capacitor 132 may be set to anysuitable value depending on the amount of voltage headroom desired(i.e., the desired voltage across the capacitor at the end of theemission state) and/or the voltage of the power supply PVSS and PVDDand/or the desired first drive current level and/or the duration of thefirst drive current and/or the duty ratio of emission state to chargingstate. By way of non-limiting example, if a first drive current 136 ofabout 10 A is desired, for a first drive current duration of about 300ps, with a desired reduction of voltage across the first capacitor 132during the emission state of about 3V, the first capacitor 132 may havea capacitance of about 3 nF.

Furthermore, because the capacitor 132 can store only a finite amount ofenergy, if there is a fault or failure in the system, the relativelyhigh first driver current 136 can only be sustained for a finite,relatively short, period of time. Consequently, the light source system100 may have improved safety, compared with other devices that drawdrive current from a less limited supply.

In order to transition the first driver 130 from the emission state tothe charging state, the FET 440 is turned off (i.e., the switch 134 isopened), thereby opening the first drive current circuit and stoppingthe first drive current 136 from flowing through the VCSEL 110. Tocontrol this transition, the controller 430 may use control lines 434and/or 436 to control operation of the FET 440. For example, thecontroller 430 may receive an instruction via its input line 432 to stopa light pulse emission from the VCSEL 110. The controller 430 may thenset control line 434 to a level that will turn off the FET 440, forexample to a voltage that is below the turn-on threshold voltage of theFET 440. The controller 430 may also apply a turn-off signal to the gateof the FET 440 using the turn-off pre-driver 422, which may be designedto have a very high drive strength and speed in one direction (i.e.,pulling down the gate of the FET 440). For example, because in thisimplementation the turn-off pre-driver 422 is an inverting typepre-driver, the controller 430 may set the voltage at control line 436to a ‘high’ voltage (e.g., 3.3V, or 5V), resulting in the output of theturn-off pre-driver 422 going ‘low’ (e.g., to 0V). Using the turn-offpre-driver 422 in this way, the speed with which the relatively highcurrent FET 440 may switch off may be improved. The inventors haverecognised that whilst this control of the turn-on pre-driver 420 andthe turn-off pre-driver 422 may take place at the same time and arelatively fast turn off speed be achieved for the FET 440, an evenfaster turn-off speed may be achieved by applying the turn-off signal tothe gate of the FET 440 first (such that both the turn-on signal andturn-off signal are applied to the gate of the FET 440 for a firstperiod of time) and then subsequently removing the turn-on signal suchthat then only the turn-off signal is applied to the gate terminal ofthe FET 440 for a second period of time. This effect may be particularlyrealised when the turn-off pre-driver 422 is designed to have arelatively high drive strength compared with the drive strength of theturn-on pre driver 420, which results in the turn-off pre-driver 422overdriving the turn-on pre-driver 420. The superior drive strength ofthe turn-off pre-driver 422 may thus result in pulling down the gate ofthe FET 440 faster than could otherwise be achieved. It may be counterintuitive to drive the gate of the FET 440 with both the turn-on andturn-off signal for a first period of time in order to turn off the FET440, but the inventors have nevertheless realised that such driving mayincrease the turn off speed of the FET 440.

It will be appreciated that each of the turn-on pre-driver 420 and theturn-off pre-driver 422 may be inverting or non-inverting type and thesignals applied by the controller 430 to the control lines 434 and 436set accordingly. Furthermore, it will be appreciated that the turn-offpre-driver 422 is optional and in an alternative it may be omittedentirety, with the FET 440 being turned off merely by changing thevoltage applied to its gate so that it is below the turn-on threshold ofthe FET 440.

FIG. 6 shows an example alternative implementation of the first driver130. This implementation is similar to that represented in FIGS. 4 and 5, but includes an additional turn-on pre-driver driver 610, anadditional control line 612 from the controller 430 and an additionalFET 620. The FET 620 is a relatively low current FET (for example, ratedat around 400 mA, compared with about 10 A for the FET 440) and acts asa VCSEL pre-bias, to enable a relatively low, sub-lasing threshold,current to flow through the VCSEL 110 before the VCSEL 110 is fullyturned on by the FET 440. As such, prior to desiring the turn-on of theVCSEL 110, the controller 612 may apply a turn-on voltage to the gate ofthe additional FET 620 (i.e., a voltage exceeding the turn-on thresholdvoltage of the additional FET 620) using the additional turn-onpre-driver 620. Once the additional FET 620 is turned on, a relativelylow current may flow through the additional FET 620 (for example, about400 mA). Because the current is relatively low, and below the lasingthreshold of the VCSEL 110, the VCSEL 110 will not yet start emittinglight. However, ideally, the current level will be only just below thelasing-threshold. When the VCSEL 110 is to be turned on, the FET 440 maybe turned on as described above so that the relatively high first drivecurrent 136 may flow through the first drive current circuit and turn onthe VCSEL 110. The additional FET 620 may then be turned off by thecontroller 430 by reducing the gate voltage at the additional FET 620,for example once the FET 440 is fully turned on. It will be appreciatedthat using the additional FET 620 in this way may speed up the timebetween the controller 430 starting to turn on the FET 440 and lightbeing emitted from the VCSEL 110, since the VCSEL 110 will already beclose to lasing at the time the FET 440 starts to switch on.

FIG. 7 shows an example of a further alternative implementation ofaspects of the light source system 110. This implementation shows asimplified implementation of the first driver 130 that includes just thefirst capacitor 132, the turn-on pre-driver 420 and the FET 440.However, it will be appreciated that the first driver 130 mayalternatively be implemented in the ways represented in FIG. 4, 5 or 6 .

In this implementation, a photodetector 710 is configured to receivelight emitted from the VCSEL 110. For example, the photodetector 710 maybe the photodetector used for ToF imaging and may receive some straylight emitted from the VCSEL 110 and reflected directly back to thephotodiode by device optics packaging, such that it detects lightemitted from the VCSEL 110 (and not just light reflected from the objectbeing imaged). Alternatively, it may be arranged in such a way as todirectly receive light emitted from the VCSEL 110. The photodetector 710is coupled to a detector 720, which is configured to output a signal 722indicative of whether or not the photodetector 710 has received lightoutput from the VCSEL 110. The controller 430 may then be configured tocontrol the operation of a charge pump 740 based on the received signal722. In particular, if the controller 430 transitions the first driver130 to the emission state and subsequently receives a signal from thedetector 720 indicating that the photodetector 710 received light outputfrom the VCSEL 110, the controller 430 may then enable the charge pump740 using control line 732. In this instance, the light source systemcan be assumed to be working correctly and the charge pump 740 will thenbe operable to charge the first capacitor 132 when the first driver 130is next in the charging state. If, however, the detector 720 does notoutput a signal 722 indicative of the photodetector 710 having receivedlight output from the VCSEL 110, the controller 430 may disable thecharge pump 740 using control line 732. The charge pump 740 may then notcharge the first capacitor 132 when the first driver 130 is next in thecharging state, such that no further energy will be stored by the firstcapacitor 132 for use in trying to drive the VCSEL 110. In thisinstance, the photodetector 710 may not have received any light emittedfrom the VCSEL 110 because there may be some error or failure in thelight source system that is preventing the VCSEL 110 from emitting lightproperly. Since the light source system may be used in safety criticalToF applications, or the fault may be of a type that would beelectrically dangerous to the light source system and/or a nearby systemand/or an operator, preventing further capacitor charging and attemptedemissions from the VCSEL 110 may improve the overall safety of the lightsource system. Optionally, the controller 430 may be configured tooutput a failure warning to any other suitable systems/entities.

Whilst a charge-pump 740 is used in this particular example, any othersuitable circuit/component may be used to controlconnection/disconnection of the first driver 130 to the power supplybased on detection of light emitted from the VCSEL 110. For example, thecharge pump 740 could be replaced with a switch (such as a transistor)controlled by the controller 430 to connect or disconnect the firstdriver 130 from the power supply PVDD. In a further alternative, ratherthan using a charge pump 740, the power supply could be a controllablepower supply, such as a switch mode power supply, which may becontrolled by the controller 430 to provide power, or not, to the firstdriver 130.

Whilst FIGS. 4 to 7 , and the above explanations, relate only to thefirst driver 130, it will be appreciated that the second driver 140 maybe implemented in exactly the same ways. The light source system 100 mayhave a single controller 430 that is configured to control the operationof the first driver 130 and the second driver 140 such that they eachgenerate the first drive current 136 and the second drive current 146 atsubstantially the same time, or each of the first driver 130 and seconddriver 140 may have their own controller that operates as describedabove. Likewise, each of the first driver 130 and the second driver 140may have a respective voltage regulator, or a single voltage regulatormay be used to power the pre-drivers in both the first driver 130 andthe second driver 140.

The skilled person will readily appreciate that various alterations ormodifications may be made to the above described aspects of thedisclosure without departing from the scope of the disclosure.

For example, whilst the above example first driver 130 includes one ormore pre-drivers 420, 422, etc, in an alternative, the pre-drivers maybe omitted entirely and the FET 440 (and optionally also additional FET620) may be driven directly by the controller 430. In this alternative,the voltage regulator 410 may be omitted entirely. Furthermore, evenwhen the first driver 130 includes one or more pre-drivers, whilst itmay be beneficial to power the pre-drivers using the voltage regulator410, at least one of the one or more pre-drivers may alternatively bepowered in any other suitable way and the voltage regulator omitted fromthe first driver 130.

Whilst in the above examples, FETs are used (for example, FET 440 andadditional FET 620), any other suitable type of controllable switch mayalternatively be used, for example any other type of transistor, such asBJTs, etc. Therefore, whenever the gate of a FET is referred to, thisshould be understood to be the gate/base of a transistor.

In the above disclosure, two drivers (the first driver 130 and thesecond driver 140) are described. Using two drivers may be particularlybeneficial for the VCSEL 110 design represented in FIGS. 1, 2A and 2B,where there are two anode terminals on the surface of the VCSEL 110, sothat the two drivers may be arranged symmetrically within the die 120.However, in an alternative, only a single driver (for example, only thefirst driver 130) may be implemented to provide all of the drive currentrequired to drive the VCSEL 110. For example, the VCSEL 110 may be of adesign where there is only one anode terminal. Alternatively, the VCSEL110 may be of a design where there are two or more anode terminals, inwhich case the single driver may be coupled to any one or more of theanode terminals.

In a further alternative, the die 120 may comprise more than twointegrated drivers arranged to drive the VCSEL 110. For example, it maycomprise four drivers, each of the same design as the first driver 130described above. These plurality of drivers may be arranged in anysuitable way within the die 110, for example first and second driversmay be symmetrical to each other with reference to a first plane ofsymmetry, and the third and fourth drivers may be symmetrical to eachother with reference to a second plane of symmetry that is perpendicularto the first plane of symmetry.

Whilst the above light source system 110 is described particularly withreference to use with ToF camera systems, the light source system 110 isnot limited to this use and may be used for any other purpose.

The terms ‘coupling’ and ‘coupled’ are used throughout the presentdisclosure to encompass both direct electrical connections between twocomponents/devices, and also indirect electrical coupling between twocomponents/devices where there are one or more intermediatecomponents/devices in the electrical coupling path between the twocomponents/devices.

Whilst the first driver 130 is described as having a switch 134 for usein controlling the transition or switching between the charging stateand emission state, it will be appreciated that the first driver 130 maybe configured in any other suitable way, using any other suitablecomponents to switch or transition the first driver 130 between acharging state, where the first capacitor 132 gradually stores chargereceived from a power supply and the VCSEL 110 is turned off, and anemission state where the first capacitor 132 discharges to generate thefirst drive current 136 to turn on the VCSEL 110. Likewise, it is notessential that a controller 430 is used to control the switching ortransition of the first driver 130 between the charging state and theemission state. Any other suitable arrangement or circuit couldalternatively be used for that purpose, for example a timer circuitconfigured to transition the first driver 130 at regular intervals, or acircuit configured to transition the first driver 130 based on theamount of charge stored on the first capacitor 132 (for example,switching to the emission state when the charge stored on the firstcapacitor 132 reaches a predetermined level), etc.

1. A circuit for coupling to a light source for controlling emission oflight from the light source, the circuit comprising: a transistor forcontrolling a drive current such that when the transistor is in a firststate, the drive current is supplied to the light source to turn thelight source on and when the transistor is in a second state, the drivecurrent is not supplied to the light source; a turn-on pre-drivercoupled to a base/gate of the transistor, wherein the turn-on pre-driveris configured to receive a turn-on control signal and output acorresponding turn-on signal to the base/gate of the transistor in orderto turn on the transistor; and a turn-off pre-driver coupled to thebase/gate of the transistor, wherein the turn-off pre-driver isconfigured to receive a turn-off control signal and output acorresponding turn-off signal to the base/gate of the transistor inorder to turn off the transistor.
 2. The circuit of claim 1, furthercomprising a controller configured to output the turn-on control signaland the turn-off control signal, wherein to turn off the transistor thecontroller is configured to: output both the turn-on control signal andthe turn-off control signal simultaneously for a first period of timesuch that both the turn-on signal and the turn-off signal are applied tothe base/gate of the transistor for the first period of time, and thenoutput only the turn-off control signal for a second period of time suchthat only the turn-off signal is applied to the base/gate of thetransistor for the second period of time.
 3. The circuit of claim 1,further comprising a capacitor for storing electrical energy for use ingenerating the drive current, wherein the circuit is configured tooperate in: a charging state when the transistor is in the second state,during which the capacitor stores charge received from a power supply;and an emission state when the transistor is in the first state, duringwhich the capacitor discharges to generate the drive current, which issupplied to the light source to turn the light source on.
 4. The circuitof claim 3, wherein the first transistor is for coupling between acathode terminal of the light source and a reference voltage, andwherein the capacitor is for coupling between an anode terminal of thelight source and the reference voltage.
 5. A light source systemcomprising: a light source; and a semiconductor die comprising anintegrated first driver, wherein the light source is mounted on asurface of the semiconductor die and the integrated first driver iscoupled to the light source and configured to control supply of a firstdrive current to the light source for controlling operation of the lightsource, wherein the integrated first driver comprises an integratedfirst capacitor for storing electrical energy for use in generating thefirst drive current and an integrated first switch for controlling thesupply of the first drive current to the light source.
 6. The lightsource system of claim 5, wherein the integrated first driver isconfigured to operate in: a charging state, during which the integratedfirst capacitor stores charge received from a power supply; and anemission state, during which the integrated first capacitor dischargesto generate the first drive current, which is supplied to the lightsource to turn the light source on.
 7. The light source system of claim6, wherein the integrated first switch is configured to: during theemission state, close a first drive current circuit comprising theintegrated first capacitor and the light source to carry the first drivecurrent between the light source and the integrated first driver; andduring the charging state, open the first drive current circuit duringthe charging state to stop the supply of first drive current to thelight source.
 8. The light source system of claim 7, wherein theintegrated first switch is coupled between a cathode terminal of thelight source and a reference voltage of the integrated first driver, andwherein the integrated first capacitor is coupled between an anodeterminal of the light source and the reference voltage of the integratedfirst driver.
 9. The light source system of claim 5, wherein theintegrated first switch comprises a first transistor.
 10. The lightsource system of claim 9, further comprising: a pre-driver configured tooutput a control signal to a gate/base terminal of the first transistorfor controlling a switch state of the first transistor.
 11. The lightsource system of claim 9, further comprising: a turn-on pre-driverconfigured to receive a turn-on control signal and output to a gate/baseterminal of the first transistor a corresponding turn-on signal in orderto turn on the first transistor and turn on the light source.
 12. Thelight source system of claim 11, further comprising a voltage regulatorcoupled to the integrated first capacitor and the turn-on pre-driver,wherein the voltage regulator is configured to: receive energy from theintegrated first capacitor; and supply a regulated voltage to theturn-on pre-driver at least during transition of the first transistorfrom an off state to an on state.
 13. The light source system of claim9, further comprising: a turn-off pre-driver configured to receive aturn off control signal and output to a gate/base terminal of the firsttransistor a corresponding turn-off signal in order to turn off thefirst transistor and turn off the light source.
 14. The light sourcesystem of claim 5, wherein the light source comprises a first terminalof a first polarity having at least one bonding contact on a firstsurface of the light source and a second terminal of a second polarityhaving at least one bonding contact on a second surface of the lightsource, and wherein the integrated first driver is connected to the atleast one bonding contact of the first terminal and to the at least onebonding contact of the second terminal such that the first drive currentcan flow through the light source to turn on the light source.
 15. Thelight source system of claim 14, wherein the first surface of the lightsource is affixed to a first surface of the semiconductor die, andwherein the first integrated driver is coupled to the at least onebonding contact of the second terminal by at least one bonding wire. 16.The light source system of claim 5, wherein the light source comprises:a first terminal of a first polarity having a first bonding contact on afirst surface of the light source; and a second terminal of a secondpolarity having a second bonding contact and a third bonding contact ona second surface of the light source, wherein the first driver iscoupled to the first bonding contact and the second bonding contact suchthat the first drive current can flow between the second terminal andthe first terminal to turn on the light source, and wherein thesemiconductor die further comprises an integrated second driver coupledto the first bonding contact and the third bonding contact, wherein thesecond driver is configured to control supply of a second drive currentto the light source such that the second drive current can flow betweenthe second terminal and the first terminal to turn on the light source.17. The light source system of claim 16, wherein the second bondingcontact and the third bonding contact are both arranged on the secondsurface of the light source such that they are substantially symmetricalabout a plane of symmetry, and wherein the first integrated driver andthe second integrated driver are arranged within the semiconductor diesuch that they are substantially symmetrical about the plane ofsymmetry.
 18. A light source control system for driving a light source,the light source control system comprising: a driver for coupling to thelight source, the driver comprising: at least one capacitor for storingcharge; and a controllable switch for switching the driver between acharging state and an emission state, wherein during the emission statethe at least one capacitor discharges to supply a drive current to thelight source to turn the light source on; a photodetector arranged todetect light emitted from the light source; and a controller coupled tothe driver and the photodetector, wherein the controller is configuredto control the driver such that during the charge state the at least onecapacitor stores charge only if the photodetector detected light emittedfrom the light source during a preceding emission state.
 19. The lightsource control system of claim 18, wherein the driver further comprisesa charge pump, wherein the controller is configured to: enable thecharge pump when the photodetector detected light emitted from the lightsource during the preceding emission state so that the charge pumpsupplies charge to the capacitor during the charging state; and disablethe charge pump when the photodetector did not detect light emitted fromthe light source during the preceding emission state so that the chargepump does not supply charge to the capacitor during the charging state.20. The light source control system of claim 18, wherein the driverfurther comprises a power switch for switchably coupling and decouplingthe driver from a power supply, wherein the controller is configured to:control the power switch to couple the capacitor to the power supplywhen the photodetector detected light emitted from the light sourceduring the preceding emission state so that charge is supplied to thecapacitor by the power supply during the charging state; and control thepower switch to decouple the capacitor from the power supply when thephotodetector did not detect light emitted from the light source duringthe preceding emission state so that charge is not supplied to thecapacitor by the power supply during the charging state.